You could try looking across the Atlantic for some of the European orphan sizes like half plate or the early 13x18 cm models that won't take modern holders. The collectable sections on eBay.de and eBay.uk have some good deals but you have to wade through a lot of junk to find them. Even at a Christies sale tailboard cameras and other simple field types don't sell for much, so check the mainstream brick-and-mortar auctions too. Otherwise, do a google on words like 'Lancaster' or 'Thornton-Pickard' to pick up the small dealers.